A new link is emerging between letters containing the poison ricin (search) found in mail facilities that serve the White House (searchand a South Carolina (searchairport as federal investigators seek to identify the letter or parcel that may have carried ricin into a Senate mailroom.

A senior law enforcement official, speaking Tuesday on condition of anonymity, said investigators had established strong links between the South Carolina and White House letters. What remained unclear, the official said, was whether those letters were connected to the substance found in the office of Senate Majority Leader Bill Frist (search), R-Tenn.

The letter found in October at a postal facility serving the Greenville-Spartanburg International Airport -- signed by someone who called himself "Fallen Angel" -- and one found in November at a facility that processes mail for the White House both complained about new regulations requiring certain amounts of rest for truck drivers, the official said. Both also contained ricin.

Investigators said Tuesday they had not identified the letter or package that might have carried ricin into Frist's office. An initial check found no extortion, threat or complaint letter in the office, said a second law enforcement source also speaking on condition of anonymity.

There were no indications of involvement by foreign terrorists such as Al Qaeda, which the FBI has said is interested in using ricin in an attack.

The powdery white substance was found on a machine that opens mail in Frist's office, authorities said. The area in the Tennessee senator's office was quarantined and stacks of mail were to be checked.

"We have an open mind about the source of this," said Terrance Gainer, chief of the U.S. Capitol Police, which is conducting the probe along with the FBI and the multi-agency joint terrorism task force based at the FBI's Washington field office.

Gainer said authorities were interviewing members of Frist's staff and others who had access to the mailroom. Although it was considered remotely possible that the ricin was physically planted in Frist's office, investigators were concentrating on mail as the likely source.

The package found in a South Carolina mail facility had a letter claiming the author could make more ricin and a threat to "start dumping" large quantities if his demands to stop the new trucking regulations were not met. The FBI offered a $100,000 reward in that case but no arrests have been made.

The White House letter, intercepted in November, contained nearly identical language but such weak amounts of ricin that it was not deemed a major health threat, said another law enforcement official. That letter's existence was not publicly disclosed before Tuesday.

At the Capitol, an FBI hazardous materials team was helping police isolate and examine the mail in Frist's office and will in the coming days collect other unopened mail in the Capitol complex, said FBI spokeswoman Debra Weierman. The FBI also will do forensic analysis at its laboratory in Quantico, Va., checking evidence for fingerprints, fibers, hair and the like.

The latest discovery comes as the FBI continues its 28-month-old investigation into the fall 2001 mailings of anthrax-laced letters to Senate and news media offices. Five people died and 17 were injured in that attack.

The anthrax investigation is ongoing, FBI spokesman Ed Cogswell said. Twenty-eight FBI agents and 12 postal inspectors are assigned full-time to the anthrax case, which has involved some 5,000 interviews and issuance of 4,000 subpoenas.

The FBI has focused recently on an intensive scientific effort to determine how the spores were made and narrow the possibilities in terms of who had the means to make them. Authorities have many theories on who might be responsible, ranging from Al Qaeda terrorists to a disgruntled scientist to an expert who sought to expose U.S. vulnerabilities to bioweapons attacks.

The one man named a "person of interest" by authorities, Dr. Steven J. Hatfill, says he has nothing to do with the attacks and has sued the government for publicly identifying him. Hatfill is a former government scientist and bioweapons expert who once worked at the Army Medical Research Institute of Infections Diseases at Fort Detrick, Md.